A Long Term Evolution (LTE) system supports two duplex modes: frequency division duplex (FDD) and time division duplex (TDD). The FDD means that uplink transmission and downlink transmission are performed on different carrier frequency bands, and both a terminal device and an access network device are allowed to simultaneously receive and send signals. The TDD means that uplink transmission and downlink transmission are performed on a same carrier frequency band, and the terminal device/the access network device performs channel sending/receiving or receiving/sending at different time points. Based on the hybrid automatic repeat request (HARQ) mechanism, feedback information of the terminal device for a downlink carrier includes an acknowledgement (ACK) and a non-acknowledgement (NACK). In an existing LTE system, a subframe interval between a subframe used by a terminal device to receive a physical downlink shared channel (PDSCH) and an uplink subframe used by the terminal device to feed back an ACK or a NACK corresponding to the PDSCH is predefined. This ensures the feedback of the ACK or the NACK by the terminal device and receiving of the ACK or the NACK by an access network device.
In an FDD system, after receiving, in a downlink subframe n−4, downlink data carried by a PDSCH, the terminal device may feed back, in an uplink subframe n, an ACK or a NACK corresponding to the PDSCH. In a TDD system, an uplink/downlink configuration of a radio frame is fixed. As shown in FIG. 1A, the LTE system currently supports seven different TDD uplink/downlink configurations. D indicates a downlink subframe. S indicates a special subframe (SSF). U indicates an uplink subframe. FIG. 1B shows a time sequence relationship or a timing relationship between a PDSCH and a corresponding ACK or NACK. A subframe marked with a number is an uplink subframe n used to feed back an ACK or a NACK. The marked number indicates that an ACK or a NACK corresponding to a PDSCH in a downlink subframe set including a downlink subframe n−k (k belongs to K) needs to be fed back in the uplink subframe n. For example, K={7, 6} when the uplink/downlink configuration is 1 and subframe number n=2 indicates that an uplink subframe n=2 is used to feed back an ACK or a NACK corresponding to PDSCHs in downlink subframes n−7 and n−6. Specifically, n−7 indicates a downlink subframe 5, and n−6 indicates a downlink subframe 6.
A spectrum deployed in a serving cell of the existing LTE system is a licensed spectrum, that is, can be used only by a network of an operator who purchases the licensed spectrum. An unlicensed spectrum (also referred to as a license-exempt spectrum) attracts increasing concern because the unlicensed spectrum does not need to be purchased and can be used by any operator. In the future, spectrum resources will be increasingly insufficient. This limits a service capability of the licensed spectrum-based LTE system. Therefore, use of the unlicensed spectrum in an LTE system is an evolution direction. The LTE system deployed on the unlicensed spectrum is referred to as an unlicensed Long Term Evolution (U-LTE) system.
Resource sharing in the unlicensed spectrum means that only restrictions on indicators such as transmit power and out-of-band leakage are specified for using a specific spectrum, and there are some additional coexistence policies and the like in some areas. Considering friendly multi-system coexistence in the unlicensed spectrum, such as coexistence of a U-LTE system with a Wireless Fidelity (WiFi) system or coexistence of U-LTE systems of a plurality of operators, restrictions of some coexistence regulations such as listen before talk (LBT) need to be followed when the unlicensed spectrum is used. Specifically, before sending a signal on a spectrum deployed in a U-LTE serving cell, an access network device or a terminal device needs to perform clear channel assessment (CCA) on the spectrum deployed in the serving cell. The access network device or the terminal device temporarily cannot send a signal on the channel once a detected receive power exceeds a threshold. The access network device or the terminal device can send a signal on the channel only after detecting that the channel is idle. In some cases, the access network device or the terminal device further needs to perform random backoff. The channel is idle and a signal can be sent on the channel only in a time period of the random backoff. Based on this, it may be learned that, for the LTE system, data transmission in the unlicensed spectrum is opportunity-based for both the access network device sending downlink data and the terminal device sending uplink data. In addition, based on the opportunity-based data transmission in the unlicensed spectrum, an uplink/downlink configuration of the U-LTE system in the unlicensed spectrum may be flexible, to adapt to uplink and downlink service requirements of the LTE system.
Based on the foregoing features of the unlicensed spectrum (the opportunity-based data transmission and the flexible uplink/downlink configuration), a predefined timing relationship between a PDSCH and a HARQ-ACK in the prior art is difficult to ensure in the unlicensed spectrum. Because the CCA mechanism is used, it cannot be ensured that the terminal device can obtain an unlicensed spectrum resource through contention in an uplink subframe determined based on the predefined timing relationship. In addition, from a perspective of a flexible uplink/downlink configuration structure, for example, for the FDD system, it cannot be ensured in the U-LTE system that a subframe which has an fixed interval of four subframes following a subframe start boundary of a downlink subframe is an uplink subframe. For another example, for the TDD system, because the U-LTE system uses the flexible uplink/downlink configuration instead of the TDD uplink/downlink configuration in FIG. 1A, the U-LTE system cannot use the timing relationship defined in FIG. 1B for uplink ACK/NACK feedback in each TDD uplink/downlink configuration.
A technical problem that needs to be resolved currently is how to ensure that the terminal device sends ACK/NACK feedback information to the access network device based on an actual scheduling situation in a U-LTE scenario.